1. Field of the Invention
The present invention relates to a taking lens and a lens-fitted film unit equipped with the taking lens.
2. Description Related to the Prior Art
A variety of photographic cameras have been on the market according to uses and functions. Further, in recent years, lens-fitted film units have been popular for the purpose of easily enjoying photography without using ordinary photographic cameras. Such a lens-fitted film unit comprises a plastic light-tight unit casing with a taking lens, an exposure mechanism including a shutter and an unexposed filmstrip that is pre-loaded. This lens-fitted film unit makes the photographer take picture immediately after purchase thereof and, after exposure of the maximum number of exposures available on a filmstrip thereof, is simply given to a photofinisher for processing and making prints. This convenience has popularized widely the lens-fitted film unit. Because one of the important advantages of the lens-fitted film unit is a low price, it is essential to employ a simple structure. Taking lenses for use with the lens-fitted film units or low price photographic cameras generally comprise one or two lens elements. Such a taking lens comprising one or two lens elements is too poor in optical performance to provide a high quality image. On this grounds, in order to remove the curvature of field of the taking lens, the lens-fitted film unit is configured such as to support a filmstrip on an image surface curved in a lengthwise direction of the image frame of a filmstrip or a horizontal direction and having a center of curvature on the object side. On the other hand, thanks to remarkable progress in processing speed, an image processing technique has been established for reading an image directly from negatives by an image scanner, and improving contrast and the tone of color of the image and correcting various aberrations through digital image processing so as to provide a high quality image.
When exposing an exposure frame of a film placed on a curved image surface, while an image is somewhat prevented from being blurred due to the curvature of field of the taking lens, it is accompanied by emphasized distortion due to the curvature of the film. When the image surface curves in the lengthwise direction of the image frame, the distortion takes barrel-shaped distortion in the lengthwise direction. In order to remove such barrel-shaped distortion, it is necessary to provide a gently curved image surface, or otherwise to design the taking lens with the intention of causing it to produce positive distortion. When providing the image surface with gentle curvature, the distortion is somewhat restrained, which is however realized at the cost for the effect of improvement of an out-of-focus image that occurs due to the curvature of field of the taking lens. Further, when the taking lens comprises a single lens element and an aperture stop disposed on the object side of the lens element, it inevitably produces positive distortion with the effect of improving the above mentioned problem. However, when employing two lens elements for the taking lens with the intention to improve optical performance, distortion that the taking lens produces tends to become negative with improvement of marginal image quality and, in particular, a modulation transfer function (MTF). Accordingly, designing the two-lens element taking lens with the intention to cause the taking lens to produce positive distortion leads to an occurrence of a reduction in MTF at marginal portions of an image and aggravation of longitudinal chromatic aberration of magnification, so that the effect of the two-lens element taking lens is made feeble. When providing the taking lens with positive distortion, pincushion-shaped distortion occurs in a transverse direction of the image frame or a vertical direction which is perpendicular to the lengthwise direction of the image surface.
As described above, an image formed by the conventional taking lens is accompanied by aggravation of image quality due to a complex effect of various aberrations and a decrease in contrast. On the grounds, if the digital image processing to correct and improve the image quality, it is necessary to implement various image processing, which needs an ample of time.
It is accordingly an object of the present invention to provide a taking lens which forms an image most suitable for digital image processing.
It is another object of the present invention to provide a lens-fitted film unit equipped with a taking lens which forms an image most suitable for digital image processing.
The above objects of the present invention are achieved by a taking lens for forming an image on a filmstrip placed on an image surface which is curved in the lengthwise direction of the image frame of a filmstrip and has a center of curvature on the object end so as to correct the curvature of field for the taking lens in the image surface. The taking lens comprises in order from the object end to the image end a meniscus first lens element having a convex object side surface and a meniscus second lens element having a convex image side surface. Letting Ep, xcex4 and Db be the axial distance between the curved image surface and an exit pupil of the taking lens which takes a value in mm greater than zero, the axial distance of opposite lengthwise ends of the curved image surface from the center of the curved image surface which takes a value in mm greater than zero, and the distortion which is equivalent but opposite in direction to distortion D occurring due to the curvature of image surface and given by (xcex4/Ep)xc3x97100%, respectively, the taking lens satisfies the following condition,
xe2x88x9210%xe2x89xa6Dsxe2x89xa6Dbxe2x88x920.5%
where
Ds is optical distortion in a plane including a paraxial focal point of light rays traveling from the exit pupil of the taking lens to a corners of the curved image surface.
The taking lens is desirably provided with an aperture stop disposed either on the image side of the meniscus second lens element or between the meniscus first lens element and the meniscus second lens element.
The taking lens preferably has aspheric surfaces at the convex object side surface of the meniscus first lens element and the convex image side surface of the meniscus second lens element. The aspheric surface may be defined by the following equation:
Z=ch2/[1+{1xe2x88x92(1+K)c2h2}xc2xd]+Ah4+Bh6+Ch8+Dh10
where
Z is the surface sag at a semi-aperture distance h from the optical axis of the taking lens;
c is the curvature of a lens surface at the optical axis equal to the reciprocal of the radius at the optical axis;
K is a conic constant;
A, B, C and D are aspheric coefficients.
The taking lens is installed to a lens-fitted film unit which comprises a light-tight unit housing to which the taking lens is fixedly attached, a film cartridge with an unexposed filmstrip which is preloaded in the unit housing and is recorded, optically or magnetically, with data relating to distortion of the taking lens for the purpose of compensating distortion of an image formed on the filmstrip by the taking lens during digital image processing.
According to the present invention, the zoom lens is structured with the design intention to produce aggravation of image quality due only to distortion and is, however, free from or significantly improved in aggravation of other aberrations, so that the digital image processing is allowed to compensate only distortion of an image for improvement of image quality.
Before describing the present invention in detail, reference is made to FIG. 1 for the purpose of providing a brief background that will enhance an understanding of an occurrence of distortion.
As schematically shown in FIG. 1, a taking lens 60 that comprises a meniscus first lens element convex to the object side and a meniscus second lens element convex to the object side forming an image on an image surface, namely an image frame 64 of a filmstrip, with light rays from an exit pupil 63 of the taking lens 60. As is well known in the art, since image bearing light rays travels from the exit pupil of the taking lens to the image surface and converge on the image surface, if the image bearing light rays are focused on a point on the image surface off from the optical axis 60a of the taking lens 60, a magnification of image depends upon a distance between the exit pupil 63 and the image surface. Distortion of an image results from variations of the magnification of image. In order to compensate the distortion of the taking lens 60, the image surface is defined as a curved surface concave to the object side in the lengthwise direction of the image frame 64. In such a case, a point on the image surface comes progressively closer to the exit pupil 63 as the point is progressively apart farther from the optical axis 60a of the taking lens 60 in the lengthwise direction. That is, the distance of the image surface from the exit pupil 63 of the taking lens 60 is not constant in the lengthwise direction and varies depending upon a distance from the optical axis 60a of the taking lens 60 in the lengthwise direction, and, in consequence, the magnification of image varies depending upon a distance from the optical axis 60a of the taking lens 60 on the image surface. As a result, although the taking lens 60 is free from distortion and is sharply focused, successive points that make up a straight line in an object are refracted progressively closer to or farther from the optical axis 60a than their ideal positions with the result that imaged lines bend inward like the edges of a pincushion or outward like the sides of a barrel.
A ratio xcex2 between a magnification at a corner of the curved image surface on which the image frame 64 is placed and a magnification at a corner of a flat focal plane 65 which is an ideal image surface on which an image of an object at an infinite distance is focused by an aberration free the taking lens can be approximated by the following equation (I):
xcex2=Yxe2x80x2/Y={(Epxe2x88x92xcex4)xc3x97tan xcex8}/(Epxc3x97tan xcex8)xe2x80x83xe2x80x83(I)
where
Y is the distance at the corner of the flat focal plane 65 from the optical axis 60a of the taking lens 60;
Yxe2x80x2 is the distance at the corner of the curved image surface from the optical axis 60a of the taking lens 60;
xcex8 is a half angle of view of the taking lens.
The distortion D in % at the corner of the curved image surface that is cause due to curvature is given by the following equation (II):
D=(Yxe2x80x2xe2x88x92Y)/Yxc3x97100%xe2x80x83xe2x80x83(II)
The equation (II) is rewritten as follows:
D=xe2x88x92xcex4/Epxc3x97100%
Since values Ep and xcex4 take positive values, the distortion is always negative, which means a barrel-shaped distortion. In the case where a negative is provided from an image frame placed in a curved image surface during exposure and is placed in a flat plane during printing, the distance at the corner of the curved image surface from the optical axis 60a of the taking lens 60 should be converted to a distance of the corner when the image surface is developed to a flat surface, these distortion D and magnification ratio xcex2 can be obtained as approximate values by the use of the distance Yxe2x80x2 at the corner of the curved image surface as mentioned above. Accordingly, when the taking lens 60 has the distortion Ds at the corner of the flat focal plane 65 including a paraxial focal point which is equivalent in value but opposite in direction to the distortion D at the corner of the curved image surface, namely Ds=xe2x88x92D, in other words, when the taking lens has positive distortion Ds, the taking lens 60 is prevented from producing a barrel-shaped distortion on the curved image surface.
On the other hand, though the image surface is curved as described above, a point on the image surface is stationary with respect to the exit pupil 63 independently from a shift of the point from the optical axis 60a of the taking lens 60 in a transverse direction (vertical direction) perpendicular to the lengthwise direction. That is, though a point on the curved image surface comes progressively closer to the exit pupil 63 with a variation of magnification as the point is progressively apart farther from the optical axis 60a of the taking lens 60 in the lengthwise direction, the change in the distance of the point from the exit pupil 63 is constant in the transverse direction, so that the curved image surface does not cause distortion of an image formed thereon in the transverse direction. Accordingly, the taking lens 60, which is designed with the intention to have the positive distortion Ds as describe above, produces pincushion-shaped distortion.
In consideration of the above discussions, distortion appearing on the curved image surface is balanced in both lengthwise and transverse directions by providing the taking lens 60 with distortion Ds which is slightly smaller than distortion Db which is equivalent to but opposite in direction to distortion D occurring due to the curvature of image surface. However, when providing the taking lens 60 with positive distortion Ds equivalent to the distortion Db focusing only on balancing distortion in the lengthwise and transverse directions, the taking lens 60 encounters aggravation of longitudinal chromatic aberration which causes a decrease in marginal contrast of the image. In the case of designing the taking lens 60 on condition that distortion accompanying the image is compensated through digital image processing, it is allowed to provide the taking lens with distortion Ds rather smaller than the distortion Db which occurs due to the curvature of image surface with the result of lowering the decrease in marginal contrast of the image due to aggravation of longitudinal chromatic aberration and, in consequence, making it possible to omit processing for improvement of marginal contrast of the image through the digital image processing. For this reason, the taking lens 60 is preferred to have distortion Ds equal to or less than distortion approximately 0.5% smaller than distortion Db that occurs due to the curvature of image surface and is suitable for balancing distortion in both lengthwise and transverse directions. On the other hand, if the taking lens 60 has distortion Ds smaller in excess than the distortion Db due to the curvature of image surface, the image at its marginal portions is enlarged at an increased ratio during processing for marginal contrast improvement, which results in aggravation of resolution of the image after digital image processing. For this reason, the taking lens 60 is preferred to have distortion Ds equal to or greater than (10%.
Data relating to the distortion Ds of the taking lens 60 is printed on the cartridge shell of the film cartridge, or otherwise may be recorded on a filmstrip in the cartridge that is pre-loaded in a lens-fitted film unit, optically or magnetically. The data is utilized to perform quick processing of compensation or removal of distortion appearing on an image formed by the taking lens during the digital image processing.
The taking lens is preferably equipped with an aperture stop which may be disposed either between the two component lens elements or behind the component lens element closer to the master image side.